In gas stations delivering liquid fuel, each of the dispensing pipes is terminated by a hose pipe equipped at one end with a gun that the user inserts into the feed pipe of the tank of the vehicle.
Naturally, as the tank fills, an equivalent volume of hydrocarbon vapor that it contains escapes to the outside via said feed pipe. In principle, vapor recovery thus consists in sucking up at the outlet of the feed pipe a volume of vapor that must at all times be equal to the delivered volume of liquid fuel. For this purpose, the dispensing gun is equipped with a sleeve which penetrates into the outlet orifice of the tank of the vehicle, and into which the upstream end of a recovery pipe opens out, on which recovery pipe a suction pump is placed which returns the recovered hydrocarbon vapor to the storage tank for storing the liquid fuel. For the recovery system to operate satisfactorily, it must be possible to vary the speed of rotation of the pump so that the instantaneous volume of sucked-up vapor equals the instantaneous volume of dispensed liquid.
With known recovery systems, of the type including a vapor suction pump with the flow rate being regulated by varying its speed of rotation, it is difficult for the volume flow rate of the liquid being supplied and the volume flow rate of the vapor being recovered to be kept equal in all circumstances because:
inevitable internal leaks exist in rotary vapor pumps, the size of the leaks increasing with wear and with the upstream-downstream pressure difference that such pumps must generate to cause the vapor to flow and to be transferred to the storage tank; PA1 it is impossible to be aware at all times of the states of the vapor recovery pipes, especially those integrated in the fuel-dispensing hose pipes, it being possible for their headloss coefficients to vary considerably over time; and PA1 the pressure in the storage tank varies.
One known way of avoiding those drawbacks consists in inserting into the recovery pipe a gas flow meter optionally associated with a pressure sensor so as to servo-control the speed of rotation of the vapor pump or the opening of a valve so as to obtain the desired vapor flow rate.
However, this method makes it necessary to implement a rapid-response servo-control loop receiving flow-rate and pressure information supplied by the measurement sensors and acting on the actuator after comparing the information with a reference value that can itself vary very rapidly over time with the flow rate of liquid fuel as controlled by the user.